In particular in the area of microlithography, apart from the use of components designed to have the highest possible precision, it is among other things desirable during operation to retain the optical modules of the imaging device (e.g. the modules with optical elements such as objectives, mirrors and grids) in such a way that they have the smallest possible deviation from a specified set position or a specified set geometry, in order to achieve a correspondingly high imaging quality (wherein within the meaning of the present disclosure the terms optical module can mean both optical elements individually and subassemblies of such optical elements and other components, such as for example holders, etc.).
In the area of microlithography the desired accuracy properties in the microscopic range are of the order of a few nanometres or less. They are not least a consequence of the constant desire to increase the resolution of the optical systems used in the manufacture of microelectronic circuits in order to drive forward the miniaturisation of the microelectronic circuits being produced. With modern lithographic systems in particular, which use a high numerical aperture in order to increase the resolution, highly polarised UV light is employed in order to be able to take full advantage of the high numerical aperture. It can be of particular importance here, therefore, to maintain the polarisation of the light when passing through the optical system. Here a particular problem that arises is the stress-induced birefringence, which is caused by stresses in the optical elements and is responsible for a significant proportion of the polarisation loss in the system. Accordingly it can be desirable to introduce as little undesired stress as possible into the optical module concerned in order to keep to a minimum the negative effects of this on the imaging quality.
One problem in this connection can arise with the creation of the connection between the optical module and the support structure which supports the optical module. The connection between the optical module and the support structure should be as rigid as possible in order to achieve the highest possible, from the dynamic point of view favourable, natural frequency.
This high rigidity of the system also can have considerable disadvantages, however. Tolerances in shape and position of the contact surfaces between the optical module and the support structure can as a general rule only be compensated for via a frictional relative movement and possibly a deformation of the components involved. Whereas the frictional relative movement leads to the introduction of parasitic shear stresses in the optical module, the deformation (as a result of the high rigidity of the system) also results in considerable parasitic stresses in the optical module. In addition, because of the high rigidity of the components involved, these parasitic stresses are only built up over a comparatively long section, so that it is possible for them to propagate well into the optically active components of the optical module where their negative influence on the imaging quality is felt particularly acutely.